SW 11 - Academics

Report
Regression with a Binary
Dependent Variable (SW Chapter 11)
1
Example: Mortgage denial and race
The Boston Fed HMDA data set
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The Linear Probability Model
Yi = b0 + b1Xi + ui
But:
!Y
· What does b1 mean when Y is binary? Is b1 =
?
!X
· What does the line b0 + b1X mean when Y is binary?
· What does the predicted value Yˆ mean when Y is binary?
For example, what does Yˆ = 0.26 mean?
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The Linear Probability Model
Yi = b0 + b1Xi + ui
E(Yi|Xi) = E(b0 + b1Xi + ui|Xi) = b0 + b1Xi
and
E(Y|X) = Pr(Y=1|X)
so Yˆ = the predicted probability that Yi = 1, given X
b1 = change in probability that Y = 1 for a given Dx:
Pr(Y = 1| X = x + Dx ) - Pr(Y = 1| X = x )
b1 =
Dx
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Example: Linear Prob Model
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Linear probability model: HMDA data
!
deny
= -.080 + .604P/I ratio
(.032) (.098)
(n = 2380)
· What is the predicted value for P/I ratio = .3?
!
Pr(
deny = 1| P / Iratio = .3) = -.080 + .604*.3 = .151
· Calculating “effects:” increase P/I ratio from .3 to .4:
!
Pr(
deny = 1| P / Iratio = .4) = -.080 + .604*.4 = .212
The effect on the probability of denial of an increase in P/I
ratio from .3 to .4 is to increase the probability by .0604, that
is, by 6.04 percentage points.
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Linear probability model: HMDA data
!
deny
= -.091 + .559P/I ratio + .177black
(.032) (.098)
(.025)
Predicted probability of denial:
· for black applicant with P/I ratio = .3:
!deny = 1) = -.091 + .559*.3 + .177*1 = .254
Pr(
· for white applicant, P/I ratio = .3:
!deny = 1) = -.091 + .559*.3 + .177*0 = .077
Pr(
· difference = .177 = 17.7 percentage points
· Still plenty of room for omitted variable bias…
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Linear probability model: Application
Cattaneo, Galiani, Gertler, Martinez, and Titiunik (2009). “Housing, Health, and Happiness.”
American Economic Journal: Economic Policy 1(1): 75 - 105
• What was the impact of
Piso Firme, a large-scale
Mexican program to help
families replace dirt floors
with cement floors?
• A pledge by governor
Enrique Martinez y
Martinez led to State of
Coahuila offering free 50m2
of cement flooring ($150
value), starting in 2000, for
homeowners with dirt floors
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Cattaneo et al. (AEJ:Economic Policy
2009) “Housing, Health, & Happiness”
X1 =
“Program
dummy” = 1
if offered
Piso Firme.
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Cattaneo et al. (AEJ:Economic Policy
2009) “Housing, Health, & Happiness”
X1 =
“Program
dummy” = 1
if offered
Piso Firme
Interpretations?
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Probit and Logit Regression
The probit & logit models satisfies this:
· 0 ≤ Pr(Y = 1|X) ≤ 1 for all levels of X and for changes in
X, such as X=# kids and increasing from 0 to 4
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Probit Regression
Pr(Y = 1|X) = F(z) = F(b0 + b1X)
· F is standard normal cumulative distribution function
· z = b0 + b1X is the “z-value” or “z-index” of probit
model
Example: Suppose b0 = -2, b1= 3, X = .4, so
Pr(Y = 1|X=.4) = F(-2 + 3*.4) = F(-0.8)
Pr(Y = 1|X=.4) = .2118554
. display normal(-0.8)
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STATA Example: HMDA data
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STATA Example: HMDA data, ctd.
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Probit regression with multiple
regressors
Pr(Y = 1|X1, X2) = F(b0 + b1X1 + b2X2)
· F is standard normal cumulative distribution function.
· z = b0 + b1X1 + b2X2 is the “z-value” or “z-index” of the
probit model.
· b1 is the effect on the z-score of a unit change in X1,
holding constant X2
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STATA Example: HMDA data
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STATA Example: HMDA data
. probit deny p_irat black, r;
Probit estimates
Log likelihood = -797.13604
Number of obs
Wald chi2(2)
Prob > chi2
Pseudo R2
=
=
=
=
2380
118.18
0.0000
0.0859
-----------------------------------------------------------------------------|
Robust
deny |
Coef.
Std. Err.
z
P>|z|
[95% Conf. Interval]
-------------+---------------------------------------------------------------p_irat |
2.741637
.4441633
6.17
0.000
1.871092
3.612181
black |
.7081579
.0831877
8.51
0.000
.545113
.8712028
_cons | -2.258738
.1588168
-14.22
0.000
-2.570013
-1.947463
-----------------------------------------------------------------------------.
scalar z1 = _b[_cons]+_b[p_irat]*.3+_b[black]*0;
.
display "Pred prob, p_irat=.3, white: " normal(z1);
Pred prob, p_irat=.3, white: .07546603
NOTES:
_b[_cons] is the estimated intercept (-2.258738)
_b[p_irat] is the coefficient on p_irat (2.741637)
scalar creates a new scalar which is the result of a calculation
display prints the indicated information to the screen
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STATA Example: HMDA data
!
Pr(
deny = 1| P / I , black )
= F(-2.26 + 2.74*P/I ratio + .71*black)
(.16) (.44)
(.08)
· Is the coefficient on black statistically significant?
· Estimated effect of race for P/I ratio = .3:
!
Pr(
deny = 1| .3,1) = F(-2.26+2.74*.3+.71*1) = .233
!
Pr(
deny = 1| .3,0) = F(-2.26+2.74*.3+.71*0) = .075
· Difference in rejection probabilities = .158 (15.8 % pts)
· Still plenty of room still for omitted variable bias…
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Probit Regression Marginal Effects
Pr(Y = 1|X) = F(z) = F(b0 + b1X1 +b2X2 + b3X3 )
ˆ
d Pr(Y
= 1 X1, X 2 , X3 )
d(X1 )
= ! (²ˆ0 + ²ˆ1 X1 + ²ˆ2 X 2 + ²ˆ3 X3 )* ²ˆ1
· F is standard normal cumulative distribution function
·
!
is the standard normal probability density function
· Marginal effect depends on all the variables, even
without interaction effects
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Probit Regression Marginal Effects
.
sum pratio;
Variable |
Obs
Mean
Std. Dev.
Min
Max
-------------+-------------------------------------------------------pratio |
1140
1.027249
.286608
.497207
2.324675
. scalar meanpratio = r(mean);
. sum disp_pepsi;
Variable |
Obs
Mean
Std. Dev.
Min
Max
-------------+-------------------------------------------------------disp_pepsi |
1140
.3640351
.4813697
0
1
. scalar meandisp_pepsi = r(mean);
. sum disp_coke;
Variable |
Obs
Mean
Std. Dev.
Min
Max
-------------+-------------------------------------------------------disp_coke |
1140
.3789474
.4853379
0
1
. scalar meandisp_coke = r(mean);
. probit coke pratio disp_coke disp_pepsi;
Iteration
Iteration
Iteration
Iteration
0:
1:
2:
3:
log
log
log
log
likelihood
likelihood
likelihood
likelihood
Probit regression
Log likelihood = -710.94858
=
=
=
=
-783.86028
-711.02196
-710.94858
-710.94858
Number of obs
LR chi2(3)
Prob > chi2
Pseudo R2
=
=
=
=
1140
145.82
0.0000
0.0930
-----------------------------------------------------------------------------coke |
Coef.
Std. Err.
z
P>|z|
[95% Conf. Interval]
-------------+---------------------------------------------------------------pratio | -1.145963
.1808833
-6.34
0.000
-1.500487
-.791438
disp_coke |
.217187
.0966084
2.25
0.025
.027838
.4065359
disp_pepsi |
-.447297
.1014033
-4.41
0.000
-.6460439
-.2485502
_cons |
1.10806
.1899592
5.83
0.000
.7357465
1.480373
------------------------------------------------------------------------------
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Probit Regression Marginal Effects
Pr(Y = 1|X) = F(z) = F(b0 + b1X1 +b2X2 + b3X3 )
ˆ
d Pr(Y
= 1 X)
d( pratio)
= ! (²ˆ0 + ²ˆ1 pratio + ²ˆ2 dispcoke + ²ˆ3disppepsi) * ²ˆ1
· Marginal effect at the means is
ˆ
d Pr(Y
= 1 X)
= ! (²ˆ0 + ²ˆ1 pratio + ²ˆ2 dispcoke + ²ˆ3 disppepsi) * ²ˆ1
d( pratio)
· Average Marginal Effect is
ˆ
= 1 Xi )
1 n d Pr(Y
=
!
n i=1 d( pratio)
1 n
!!!!!!!!!!! ! ! (²ˆ0 + ²ˆ1 pratioi + ²ˆ2 dispcokei + ²ˆ3disppepsii ) * ²ˆ1
n i=1
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Logit Regression
Pr(Y = 1|X) = F(b0 + b1X)
F is the cumulative standard logistic distribution function:
1
where F(b0 + b1X) =
1 + e - ( b 0 + b1 X )
Example:
b0 = -3, b1= 2, X = .4,
so b0 + b1X = -3 + 2*.4 = -2.2 so
Pr(Y = 1|X=.4) = 1/(1+e–(–2.2)) = .0998
Why bother with logit if we have probit?
· Historically, logit is more convenient computationally
· In practice, logit and probit are very similar
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STATA Example: HMDA data
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Predicted probabilities from estimated probit and logit
models usually are (as usual) very close in this application.
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Logit Regression Marginal Effects
Pr(Y = 1|X) = F(z) = F(b0 + b1X1 +b2X2 + b3X3 )
ˆ
d Pr(Y
= 1 X)
d( pratio)
= f (!ˆ0 + !ˆ1 pratio + !ˆ2 dispcoke + !ˆ3disppepsi)* !ˆ1
· F is logistic cumulative distribution function
·
f is the logistic probability density function
e! x
f (x) =
,! ² < x < ²
!x 2
·
(1+ e )
· Marginal effect depends on all the variables, even
without interaction effects
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Comparison of Marginal Effects
LPM
Probit
Logit
Marginal Effect at Means
for Price Ratio
-.4008
(.0613)
-.4520
(.0712) via
-.4905
(.0773)
Average Marginal Effect
of Price Ratio
-.4008
(.0613)
-.4096
-.4332
(beyond eco205)
(beyond eco205)
Marginal Effect at Means
for Coke display dummy
.0771
(.0343)
.0856
(.0381)
.0864
(.0390)
Average Marginal Effect
For Coke display dummy
.0771
(.0343)
.0776
.0763
(beyond eco205)
(beyond eco205)
nlcom
via nlcom
via nlcom
via nlcom
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Probit model: Application
Arcidiacono and Vigdor (2010). “Does the River Spill Over? Estimating the Economic Returns
to Attending a Racially Diverse College.” Economic Inquiry 48(3): 537 – 557.
• Does “diversity
capital” matter and
does minority
representation
increase it?
• Does diversity
improve postgraduate outcomes
of non-minority
students?
• College & Beyond
survey, starting
college in 1976
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Arcidiacono & Vigdor (EI, 2010)
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Arcidiacono & Vigdor (EI, 2010)
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Arcidiacono & Vigdor (EI, 2010)
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Logit model: Application
Bodvarsson & Walker (2004). “Do Parental Cash Transfers Weaken Performance in College?”
Economics of Education Review 23: 483 – 495.
• When parents pay for tuition & books does this
undermine the incentive to do well?
• Univ of Nebraska @ Lincoln & Washburn Univ in Topeka,
KS, 2001-02 academic year
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Bodvarsson & Walker (EconEduR,2004)
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Bodvarsson & Walker (EconEduR,2004)
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Estimation and Inference in Probit
(and Logit) Models
Probit model:
Pr(Y = 1|X) = F(b0 + b1X)
· Estimation and inference
· How can we estimate b0 and b1?
· What is the sampling distribution of the estimators?
· Why can we use the usual methods of inference?
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Probit estimation by
maximum likelihood
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Special case: probit MLE with no X
36
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The likelihood is the joint density, treated as a function of the
unknown parameters, which here is p:
n
n
n
Y
å i =1Yi )
(
å
i =1 i
f(p;Y ,…,Y ) = p
(1 - p )
1
n
The MLE maximizes the log likelihood.
ln[f(p;Y1,…,Yn)] =
(å Y ) ln( p) + (n - å Y ) ln(1 - p)
n
n
i =1 i
i =1 i
Set the derivative = 0:
n
n
æ -1 ö
1
d ln f ( p;Y1 ,..., Yn )
= å i =1Yi
=0
+ n - å i =1Yi ç
÷
p
dp
è1- p ø
Solving for p yields the MLE; that is, pˆ MLE satisfies,
(
) (
)
38
39
The MLE in the “no-X” case
(Bernoulli distribution), ctd.:
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The MLE in the “no-X” case
(Bernoulli distribution), ctd:
· The theory of maximum likelihood estimation says that
pˆ MLE is the most efficient estimator of p – of all possible
estimators – at least for large n.
o much stronger than the Gauss-Markov theorem
· STATA note: to emphasize requirement of large-n, the
printout calls the t-statistic the z-statistic; and instead of
the F-statistic, it’s called a chi-squared statistic (= q*F).
· Now we extend this to probit – in which the probability is
conditional on X – the MLE of the probit coefficients.
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The probit likelihood with one X
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The probit likelihood function:
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The Probit MLE, ctd.
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The logit likelihood with one X
· The only difference between probit and logit is the
functional form used for the probability: F is replaced
by the cumulative logistic function (see SW Appendix
11.2)
· As with probit,
· bˆ0MLE , bˆ1MLE are consistent
· bˆ0MLE , bˆ1MLE are normally distributed
· standard errors computed by STATA
· testing & confidence intervals proceed as usual
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Measures of fit for logit and probit
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Application to the Boston HMDA
Data (SW Section 11.4)
· Mortgages (home loans) are an essential part of buying a
home.
· Is there differential access to home loans by race?
· If two otherwise identical individuals, one white and one
black, applied for a home loan, is there a difference in
the probability of denial?
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The HMDA Data Set
· Data on individual characteristics, property
characteristics, and loan denial/acceptance
· The mortgage application process circa 1990 -1991:
· Go to a bank or mortgage company
· Fill out an application (personal+financial info)
· Meet with the loan officer
· Then the loan officer decides – by law, in a race -blind
way. Presumably, the bank wants to make profitable
loans, and the loan officer doesn’t want to originate
defaults.
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The loan officer’s decision
· Loan officer uses key financial variables:
· P/I ratio
· housing expense-to-income ratio
· loan-to-value ratio
· personal credit history
· The decision rule is nonlinear:
· loan-to-value ratio > 80%
· loan-to-value ratio > 95% (what happens in default?)
· credit score
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Regression specifications
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51
52
53
Table 11.2, ctd.
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Table 11.2, ctd.
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Summary of Empirical Results
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